This invention relates to semiconductor devices and methods. The invention has particularly advantageous application to III-V semiconductor light emitting devices, including semiconductor lasers and semiconductor light emitting diodes (LEDs).
In the U.S. Pat. No. 5,936,266 of Holonyak et al. there are disclosed III-V semiconductor devices and methods in which the amount of p-type material can be minimized, with attendant advantages in electrical, thermal, and optical performance, and in fabrication. A form of that invention is directed to a generally planar semiconductor device wherein a layer of p-type semiconductor material is disposed over (that is, directly on or with one or more intervening layers) a layer of n-type semiconductor material, and an electric potential is coupled between the p-type layer and the n-type layer, and wherein current in the device that is lateral to the plane of the layers is coupled into the p-type layer. A tunnel junction is provided adjacent the p-type layer for converting the lateral current into hole current. The tunnel junction can be an n+/p+ junction oriented with the p+ portion thereof adjacent said p-type layer. The lateral current can be electron current in the n+ layer and/or electron current in a further layer of n-type material disposed over the tunnel junction. An objective of the referenced invention was to minimize the amount of p-type material (with low mobility hole conduction) and, to the extent possible, employ only n-type layers (electron conduction) to carry the device current. In addition to electrical and thermal performance advantages from reducing the amount of lossier p-type material, optical advantage can also accrue since p-type material of the same conductance as n-type material will generally be more absorptive of the light being generated in semiconductor light emitting devices. Conversely, it was noted that since the tunnel contact junction is highly doped, it should be kept relatively thin to avoid undue light absorption.
It is among the objects of the present invention to provide devices of the general type described above, but with novel structural modification that can result in further operational improvement.
A form of the invention set forth in the above-referenced U.S. Pat. No. 5,936,266 utilizes a quantum well lower gap barrier layer, for example of InyGa1xe2x88x92yAs, between the p+ GaAs and n+ GaAs layers to reduce energy gap and carrier mass, and increase tunneling probability.
In accordance with a feature hereof, the thin InxGa1xe2x88x92xAs layer can be non-continuous. The InxGa1xe2x88x92xAs layer could be xe2x80x9ccut upxe2x80x9d into parallelepiped boxes or quantum boxes or quantum dots (as in quantum dot injection devices, e.g., lasers) but, for the present purpose, to provide an array of thin tiny box-like or dot-like tunnel barriers. An electron tunnels with high probability via the dot. Thus, for example, in a VCSEL driven with lateral electron current, sufficient injection is expected (via the tunnel dots or islands) for VCSEL operation, but with reduced absorption of the recombination radiation (photons) in the laser operation directed vertically through the tunnel region. Thus, a tunnel contact of reduced absorption is possible in spite of the lower gap of the quantum dot barriers.
Quantum dot injection devices, such as quantum dot lasers, are known in the art, such as for the purpose of lowering threshold currents in laser diodes. As described, for example, in K. Eberl, xe2x80x9cQuantum Dot Lasersxe2x80x9d, Physics World, September, 1997, a quantum dot confines the motion of electrons in all three spatial directions such that electronic states are squeezed into discrete transition energies so that fewer carriers are needed to create a population inversion. Temperature dependence is also reduced due to the discrete energy spectrum and large separation between energy levels. For examples of quantum dot devices and techniques, see H. Saito et al., xe2x80x9cRoom Temperature Lasing Operation Of A Quantum Dot Vertical Cavity Surface Emitting Laserxe2x80x9d, Appl. Phys. Lett. 69 (21), Nov. 18, 1996; M. Maximov et al., xe2x80x9cHigh Power Continuous Wave Operation InGaAs/AlGaAs Quantum Dot Laserxe2x80x9d, J. Appl. Phys., 83, 10, May, 1998.
In the present invention, the non-continuous layer, in the form of quantum dots or other forms, is used to advantage in the barrier layer of a tunnel junction of a device that employs lateral electron flow into the tunnel junction.
In accordance with an embodiment of the invention, there is provided a semiconductor light emitting device that includes: a semiconductor active region disposed between first and second semiconductor layers, the first semiconductor layer being p-type, and the second semiconductor layer being n-type; tunnel junction means disposed over the first semiconductor layer, the tunnel junction means including a tunnel barrier region that is a non-continuous layer; means for coupling electric potential between the tunnel junction means and the second semiconductor layer; and means for causing lateral electron flow into the tunnel junction means.
In a preferred embodiment of the invention, the tunnel junction means comprises an n+/p+ junction oriented with the p+ portion thereof adjacent the first semiconductor layer, and wherein the n+/p+ junction includes the tunnel barrier region that is a non-continuous layer. In this embodiment, the n+/p+ junction is a GaAs junction modified with an InxGa1xe2x88x92xAs tunnel barrier region which is the non-continuous layer. The non-continuous layer can be in the form of a pattern or can be stochastic in form and/or arrangement.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.